A Syrian opposition activist group says 49 Syrian soldiers have been killed in an advance by Islamic State militants near the historic city of Palmyra.

The Britain-based Syrian Observatory for Human Rights said on Friday that the Islamic State group advanced along several fronts around Palmyra in central Syria, driving back government forces.

The IS group's Aamaq News Agency distributed video showing what it says were Syrian soldiers fleeing their positions in the badlands west of Palmyra.

The Syrian government, backed by Russian forces, recaptured Palmyra from the extremists in March, to great fanfare. The militants had destroyed numerous monuments dating back to Roman times during their hold of the city.

The two militaries have since diverted their attention to fighting local opposition forces around Damascus and Aleppo.

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1:30 p.m.

The U.N. human rights office says it has received reports that hundreds of men have gone missing after crossing from rebel-held eastern Aleppo into government-controlled areas of the Syrian city.

Spokesman Rupert Colville said Friday that family members have reported losing contact with the men, who are between the ages of 30 and 50, after they fled opposition-held areas of Aleppo around a week or 10 days ago.

Colville says the U.N. High Commissioner for Human Rights is also concerned by reports that some civilians trying to flee are being blocked by armed opposition groups and in some cases being fired upon.

In a briefing in Geneva, he said "civilians are being used as pawns and prevented from leaving."

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9:30 a.m.

The Russian military says it has helped more than 8,000 civilians leave the rebel-controlled eastern part of Aleppo.

The military's Center for Reconciliation in Syria said early Friday that 8,461 civilians, including 2,934 children, have left Aleppo's eastern neighborhoods in the last 24 hours. The center said 14 militants surrendered their weapons and were granted amnesty.

The statement came hours after Russian Foreign Minister Sergey Lavrov said that Syrian government troops suspended their active operations in Aleppo to allow the evacuation of civilians. Syrian President Bashar Assad's forces have retaken large parts of eastern Aleppo in recent days.

The Russian military said its sappers have cleared mines from 6 hectares (15 acres) of Aleppo, allowing the restoration of a water facility, two power stations, two mosques and two schools.